History of the Theory of Heredity. 75 



freely with tliefemulo of tlie second species, while absolute 

 sterility follows the union of a male of the se'cond 

 species with a female of the first species. The offspring 

 of a male hybrid and the female of a pure species is 

 much more variable than the offspring of a female 

 hybrid and the male of a pure species. These facts are 

 absolutely inexplicable, if the two sexual elements play 

 similar parts in heredity. 



A structure which is more developed or of more func- 

 tional importance in the male parent than it is in the 

 female parent is very much more apt to vary in the off- 

 spring tlian a part which is more developed or more 

 important in the mother than it is in the father. 



These facts, and many others which will be mentioned 

 farther on, compel us to believe that. there is some pro- 

 found functional difference between the ovum and the 

 male cell. 



It is, therefore, only reasonable to distrust the abso- 

 lute correctness and completeness of any hypothesis of 

 heredity, which, like Darwin's Pangenesis hypothesis, 

 recognizes no such difference. 



Summary of last two Chapters, 



The phenomena of heredity are certainly among the 

 greatest marvels of the material universe, but there is 

 no reason to believe that they lie outside the province of 

 legitimate scientific inquiry. Our present purpose is 

 not to trace them back to their origin or to show that 

 they result from the properties of matter, but simply ac- 

 cepting them as vital phenomena, to trace the secondnry 

 laws to which their present form is due. The fact that 

 the distinctive properties of the egg of any living 

 species have been gradually acquired during the evolu- 

 tion of the race through the action of influences which 



